U.S. Ser. No. 07/712,269 now U.S. Pat. No. 5,110,070, issued on May 5, 1992 (Hagenlocher et al.) discloses a rigid airship in which triangular cross-ribs are interconnected by longitudinal beams which together with the cross-ribs form a carrier frame enclosed by an envelope or skin. Carrier gas cells and other structural components of the airship are mounted within the carrier frame which is divided into longitudinal sections, each of which is formed by two cross-ribs and three longitudinal beam elements. Each cross-rib is formed by three rib elements, at least two of which are of equal length forming a isosceles triangle, the tip of which points upwardly, and the base of which extends near the bottom of the airship horizontally. The corners of the isosceles triangles are interconnected by the beam elements forming the longitudinal beams. The rectangular areas bounded by two rib elements and by two beam elements are reinforced by diagonally extending tensioning elements, for example, a cable with a turnbuckle. Inflatable and deflatable air chambers are provided for steering purposes in addition to inflatable lifting gas cells.
The features disclosed in U.S. Ser. No. 07/712,269 (Hagenlocher et al.) combine the advantages of a rigid airship with the advantages of a blimp, while avoiding the disadvantages of both types of structures. Such advantages are desirable because, in spite of the great advances that have been made in aircraft constructions, there are several fields of application in which the use of an airship is much more advantageous than the use of an aircraft, such as a helicopter. Such advantages are especially important, for example, when an airship must cruise at a low speed over the destination after arrival or where even a prolonged localized hovering may be required for the accomplishing of a specific task.
Airships with a rigid frame are usually referred to as rigid airships which have certain technical advantages over blimps, especially with regard to their steerability. Nevertheless, rigid airships are frequently not used because they are substantially more expensive than non-rigid pressurized airships. Thus, instead of using a rigid airship, non-rigid pressurized airships are used because they are substantially smaller and less expensive.
The construction of a rigid airship requires the formation of a trusswork that includes the cross-ribs and longitudinal spars or beams. Such a construction is labor intensive and additionally results in a substantial weight of the carrier frame. These structural efforts and expenses frequently outweigh the advantages of a rigid airship, namely its steerability, even if there is a pressure drop within the envelope and its high travel speed and its low fuel consumption. Thus, the construction of rigid airships heretofore was feasible only in an economical sense if the total carrier volume of the ship was exceeding about 25,000 cubic meter. For these reasons, non-rigid pressurized airships have been used in practice heretofore.
However, pressurized airships have substantial disadvantages. When a pressure drop occurs in the pressurized envelope or skin, such blimp-type airship is no longer steerable. Additionally, due to the pressurization of the envelope, it is not practically possible to maintain an aerodynamic shape. The resulting higher drag values for pressurized airships dictate a smaller travelling speed combined with higher fuel consumption. Moreover, the use of pressurized airships is very much dependent on whether conditions such as strong wind or thunderstorms which limit the use of pressurized airships in practice. Still another disadvantage is seen in that due to the lack of a carrier frame it is necessary to mount the propulsion plants to the gondola, whereby the noise and vibration level in the gondola is quite substantial.
As mentioned above, the construction of a rigid airship as disclosed by Hagenlocher et al. wants to combine the advantages of both types of airships while avoiding their disadvantages. The carrier frame of Hagenlocher et al. has a substantially reduced weight so that it becomes feasible for use in rigid airships having a smaller carrier gas volume than was conventionally possible or economically feasible. Thus, Hagenlocher et al. make it possible to incorporate the positive characteristics of rigid airships into such ships with substantially smaller carrier gas volumes. In fact, the structural weight of rigid airships according to Hagenlocher et al. can be reduced to about fifty percent of comparable conventional rigid airships. A carrier gas volume of about 12,000 cubic meter is quite feasible for a rigid airship constructed as disclosed by Hagenlocher et al. while simultaneously achieving the desirable advantages of a rigid airship, for example, an aerodynamic shape, high cruising speeds, and low fuel consumptions with an increased safety and continued steerability, even when there is a pressure loss in a carrier gas cell.
The outer contour in conventional rigid airships is determined by ring-shaped or polygonal cross-ribs. This conventional feature requires that the skin or envelope is applied to the circumference of the frame in the form of individual ring webs which then must be stretched. Once the ring webs are applied to the frame, circumferential seams must be formed to interconnect the ring webs. A plurality of carrier gas cells are arranged inside the finished skin or envelope. Such a construction of the skin out of ring-shaped webs has its disadvantages, especially for the intended combination of the features of a rigid airship with those of a pressurized airship, whereby only a single carrier gas cell is present, namely that formed by the outer envelope. The mounting of ring-shaped envelope webs or portions onto the triangular structure of the carrier frame is hard to accomplish especially if it is necessary that the finished envelope shall have a circular cross-section to form the inflatable carrier gas envelope. Another disadvantage of such a skin construction made of a plurality of ring webs resides in the fact that the accessibility to the interior of the airship, for example, for maintenance purposes, is made more difficult by the circumferentially closed ring webs.
It is customary to use so-called ballonets in pressurized airships. These ballonets are air chambers that can be inflated and deflated as required for steering purposes. However, such air chambers for steering purposes must be arranged inside the carrier gas envelope, whereby the steering air chambers are conventionally secured to that portion of the carrier gas envelope which faces the airship gondola. This type of installation of the steering air chambers is not possible in a carrier frame as suggested above, due to the construction of the carrier frame.